You guys are going on like you actually want to make it complicated. If Cheekz has to have a HU, then get a Alpine with the AI-net controller built in. Then run the SPDIF from the PC sound card to the H700 brain. Then use the HU to switch. That seems much simpler to me than making and modding a bunch of stuff.
BTW, I doubt that Turtle beach USB is going to have high sound quality. Might be OK, but I wouldn't use it.